1. Field of the Invention
This invention relates generally to a transparent structure through which light rays may be passed and which is useful to form a photographic mask, or plate, which is ultimately used in connection with known chemical etching processes for forming three-dimensional designs on metal rolls, plates, and the like.
More specifically this invention relates to a two-layer, transparency having variations in optical density corresponding to deviations of a surface contour from a plane of an original surface, a method for forming it, and to the use of the transparency in making a mask or plate having similar variations in optical density after development.
2. Description of Prior Art
In the fabrication of embossing rolls or plates, the replication of a master or original model and the conversion of the replica into a photographic transparency or overlay is an essential part of the fabrication procedures.
It has been known in the past to photograph various decorative patterns and objects having contoured surfaces, and to utilize known photographic and chemical milling techniques to reproduce the configurations of the original surfaces on metal rolls, plates, etc. U.S. Pat. No. 1,548,465 is an example of such a process used for the reproduction of a wood grain on metal surfaces. Another photographic and chemical milling process for producing an embossing cylinder is disclosed by U.S. Pat. No. 3,775,261. U.S. Pat. No. 3,711,346 discloses a method for reproducing a three-dimensional surface configuration in metal by photographing the contoured surface a plurality of times as the light source is elevated a few degrees at a time from a startig position at an angle of 1 to 10 degrees above the plane of the surface. The sets of developed films or masks thus generated are then used in a series of steps to chemically etch the configuration of the original surface in metal. U.S. Pat. No. 2,854,336 relates to a method of forming a two-level photo-engraved embossing plate or mold comprising the use of two different negatives to form a primary design and a background design on metal using known photographic and chemical milling techniques.
The prior art attempts to faithfully reproduce in metal the various three-dimensional contours of an original surface were somewhat less than satisfactory. Primarily this was due to the previous difficulty of obtaining on the developed films or masks, the varying optical densities required to enable the surface contours of an original surface to be replicated on metal by known chemical milling procedures.